1. Field of the Invention
The present invention relates to a water-decomposable fibrous sheet capable of being readily decomposed and dispersed in water flow. More precisely, it relates to a water-decomposable fibrous sheet having high strength in dry and wet but capable of being readily decomposed in water.
2. Description of the Related Art
To wipe the skin of human bodies including the private parts thereof, or to clean toilets and thereabouts, used are cleaning sheets made of paper or non-woven fabric. The cleaning sheets must be decomposable in water so that they could be directly disposed of in toilets after their use. This is because, if hardly water-decomposable cleaning sheets are disposed of in toilets after their use, they will take a lot of time until they are decomposed and dispersed in septic tanks, or will clog the drainpipes around toilets.
For easy and effective use, many disposable cleaning sheets for wiper applications are packaged while being wetted with a detergent chemical or the like, and are put on the market. Such cleaning sheets must have high strength in wet to such a degree that they are well fit for wiping with them containing a detergent chemical or the like, but must well decompose in water after they are disposed of in toilets.
For example, Japanese Patent Publication No. 24636/1995 discloses a water-decomposable cleaning article that comprises a carboxyl group-having, water-soluble binder, a metal ion and an organic solvent. However, the metal ion and the organic solvent irritate the skin.
Japanese Patent Laid-Open No. 292924/1991 discloses a water-decomposable cleaning article of polyvinyl alcohol-containing fibers with an aqueous solution of boric acid infiltrated thereinto; and Japanese Patent Laid-Open No. 198778/1994 discloses a water-decomposable napkin of polyvinyl alcohol-containing non-woven fabric with a borate ion and a bicarbonate ion introduced thereinto. However, polyvinyl alcohol is not resistant to heat, and therefore the wet strength of the water-decomposable cleaning article and the water-decomposable napkin is lowered at 40xc2x0 C. or higher. Recently, various water-decomposable absorbent articles including sanitary napkins, panty liners, disposable diapers and others have been investigated in the art. In view of their safety, however, the water-decomposable fibrous sheets mentioned above could not be used as the top sheets for those absorbent articles that shall be kept in direct contact with the skin for a long period of time, as they contain a binder and an electrolyte.
On the other hand, Japanese Patent Laid-Open No. 228214/1997 discloses a water-degradable non-woven fabric having a wet strength of from 100 to 800 gf/25 mm as measured according to JIS P-8135, which is produced by mixing fibers having a length of from 4 to 20 mm with pulp followed by entangling them through treatment with high-pressure water jets. Since the constituent fibers are entangled in it, the non-woven fabric disclosed has a bulky feel. However, in producing the non-woven fabric, long fibers are entangled through high-pressure water jet treatment, whereby the non-woven fabric produced could have such a relatively high wet strength. Therefore, according to the technique disclosed, it is difficult to realize well-balanced bulkiness, strength and water-degradability for the non-woven fabric produced, and the non-woven fabric produced is unsuitable to disposal in flush toilets, etc.
The present invention is to solve the problems in the prior art noted above, and its one object is to provide a water-decomposable fibrous sheet which is well decomposed in water and has high dry strength.
Another object of the invention is to provide a water-decomposable fibrous sheet which has high wet strength to such a degree that it is well usable in wet even though no binder is added thereto.
Still another object of the invention is to provide a water-decomposable fibrous sheet which is safe for its application to the skin.
Specifically, the invention is to provide a water-decomposable fibrous sheet, which comprises from 3% by mass to 100% by mass of fibrillated rayon comprising primary fibers and microfibers extending therefrom, and from 0% by mass to 97% by mass of other fibers having a length of at most 10 mm, and in which the fibrillated rayon has a degree of beating of at most 700 cc; the primary fibers have a length in a range of from 1.8 mm to 10 mm at a peak of mass distribution thereof; and at least the microfibers extending from the primary fibers of the fibrillated rayon are entangled with at least one of other primary fibers, other microfibers extending from the other primary fibers and the other fibers.
The invention is also to provide a water-decomposable fibrous sheet, which comprises from 3% by mass to 100% by mass of fibrillated rayon comprising primary fibers and microfibers extending therefrom, and from 0% by mass to 97% by mass of other fibers having a length of at most 10 mm, and in which the primary fibers have a length in a range of from 1.8 mm to 10 mm at a peak of mass distribution thereof, the microfibers having a length of at most 1 mm account for from 0.1 to 65% by mass of the self-weight of the fibrillated rayon; and at least the microfibers extending from the primary fibers of the fibrillated rayon are entangled with at least one of other primary fibers, other microfibers extending from the other primary fibers and the other fibers.
The water-decomposable fibrous sheets described above are preferably non-woven fabrics subjected to water jetting treatment.
Also, the invention is to provide a water-decomposable fibrous sheet, which comprises from 3% by mass to 100% by mass of fibrillated rayon comprising primary fibers and microfibers extending therefrom, and from 0% by mass to 97% by mass of other fibers having a length of at most 10 mm, and in which the fibrillated rayon has a degree of beating of at most 700 cc; the primary fibers have a length in a range of from 1.8 mm to 10 mm at a peak of mass distribution thereof; and at least the microfibers extending from the primary fibers of the fibrillated rayon are hydrogen bonded with at least one of other primary fibers, other microfibers extending from the other primary fibers and the other fibers.
The invention is also to provide a water-decomposable fibrous sheet, which comprises from 3% by mass to 100% by mass of fibrillated rayon comprising primary fibers and microfibers extending therefrom, and from 0% by mass to 97% by mass of other fibers having a length of at most 10 mm, and in which the primary fibers have a length in a range of from 1.8 mm to 10 mm at a peak of mass distribution thereof; the microfibers having a length of at most 1 mm account for from 0.1 to 65% by mass of the self-weight of the fibrillated rayon; and at least the microfibers extending from the primary fibers of the fibrillated rayon are hydrogen bonded with at least one of other primary fibers, other microfibers extending from the other primary fibers and the other fibers.
The water-decomposable fibrous sheets described above are preferably produced in a paper-making process. In this case, preferably, the fibrillated rayon has a degree of beating of at most 400 cc.
Naturally in dry and even in wet with water, the water-decomposable fibrous sheet of the invention all the time keeps high strength while it is used as a wiper. In addition, when it is immersed in a large amount of water after used, it is readily decomposed. Therefore, after used, it can be disposed of in toilets, etc. What is more, the water-decomposable fibrous sheet of the invention is composed of materials not harmful to human bodies.
More specifically, in the water-decomposable fibrous sheet of the invention, because the microfibers of the fibrillated rayon act to bind the fibers together, well balanced decomposability in water and strength are realized. With the microfibers entangled with or hydrogen bonded with other fibers, the fibrous sheet procures high strength. On the other hand, when kept in contact with a large amount of water, the microfibers are separated from the other fibers, and therefore, the fibrous sheet is readily decomposed in water. In particular, when the microfibers extending from the primary fibers of the fibrillated rayon are entangled with at least one of other primary fibers, other microfibers extending from the other primary fibers and the other fibers through the water jetting treatment, the fibers are strongly bound together, and moreover, the dry strength of the sheet is increased owing to the hydrogen bonding power of the microfibers. Such hydrogen bonding may sometimes be canceled in a wet condition, but the sheet can maintain high strength even in wet because of the entanglement of the microfibers.
On the other hand, when the water-decomposable fibrous sheet of the invention is produced for example in a paper-making process i.e., produced without subjecting it to water jetting treatment, the fibrous sheet has high strength owing to the presence of the microfibers. The microfibers can exhibit the hydrogen bonding power as much as, or more than pulp, and therefore, the fibrous sheet has well balanced decomposability in water and strength. The fibrous sheet thus produced in a paper-making process will be excellent in strength upon use in a dry condition. Even in such sheet, additionally, the wet strength could be increased owing to the entanglement of the microfibers.
In the invention, where the primary fibers have a length of from 2.5 mm to less than 4.5 mm at a peak of mass distribution thereof and the fibrillated rayon has a degree of beating of smaller than 400 cc, it is desirable that the microfibers having a length of at most 1 mm account for from 0.5 to 15% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of from 2.5 mm to less than 4.5 mm at a peak of mass distribution thereof and the fibrillated rayon has a degree of beating of from 400 cc to 700 cc, it is desirable that the microfibers having a length of at most 1 mm account for from 0.1 to 5% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of from 4.5 mm to 7.5 mm at a peak of mass distribution thereof and the fibrillated rayon has a degree of beating of smaller than 400 cc, it is desirable that the microfibers having a length of at most 1 mm account for from 8 to 65% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of from 4.5 mm to 7.5 mm at a peak of mass distribution thereof and the fibrillated rayon has a degree of beating of from 400 cc to 700 cc, it is desirable that the microfibers having a length of at most 1 mm account for from 0.3 to 50% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of 3 i 0.5 mm at a peak of mass distribution thereof, it is desirable that the microfibers having a length of at most 1 mm account for from 0.1 to 10% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of 4xc2x10.5 mm at a peak of mass distribution thereof, it is desirable that the microfibers having a length of at most 1 mm account for from 1 to 14% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of 5xc2x10.5 mm at a peak of mass distribution thereof, it is desirable that the microfibers having a length of at most 1 mm account for from 0.3 to 45% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of 6xc2x10.5 mm at a peak of mass distribution thereof, it is desirable that the microfibers having a length of at most 1 mm account for from 5 to 50% by mass of the self-weight of the fibrillated rayon.
Where the primary fibers have a length of 7xc2x10.5 mm at a peak of mass distribution thereof, it is desirable that the microfibers having a length of at most 1 mm account for from 10 to 65% by mass of the self-weight of the fibrillated rayon.
In the process of forming fibrillated rayon by beating rayon, the length of primary fibers of the fibrillated rayon may sometimes varies to be shorter or longer due to the beating process. In addition, the non-fibrillated rayon (rayon before beating) per se has a deviation in length. Therefore, in the above, such variation and deviation in fiber length has been taken into consideration. Where the length of the rayon before beating is 3 mm, 4mm, 5 mm, 6 mm or 7 mm, for example, the length of the primary fibers at a peak of mass distribution thereof falls in the range of 3xc2x10.5 mm, 4 xc2x10.5 mm, 5xc2x10.5 mm, 6xc2x10.5 mm or 7xc2x10.5 mm.
Where the ratio of the weight of the microfibers having a length of at most 1 mm to the self-weight of the fibrillated rayon is defined as described above, the fineness of the fibrillated rayon is preferably from 1.2 to 1.9 dtex.
Preferably, the fibers having a length of at most 10 mm are biodegradable fibers. It is desirable that the biodegradable fibers are those of at least one selected from the group consisting of regenerated cellulose, pulp, aliphatic polyesters, polyvinyl alcohol and collagen.
Preferably, the basis weight of the water-decomposable fibrous sheet of the invention falls between 20 and 100 g/m2.
Preferably, the degree of decomposition in water of the fibrous sheet is at most 200 seconds, as measured according to JIS P-4501.
Preferably, the wet strength of the fibrous sheet is at least 110 g/25 mm.
Preferably, the dry strength of the fibrous sheet is at least 350 g/25 mm.
The invention also provides a water-decomposable fibrous sheet, which comprises from 3 to 100% by mass (preferably, 5 to 100% by mass) of fibrillated rayon of such that the primary fibers constituting it have a length of from 1.8 to 10 mm, and from 0 to 97% by mass (preferably, 0 to 95% by mass) of other fibers having a length of at most 10 mm, and which has a basis weight of from 20 to 100 g/m2, a thickness of at least 0.2 mm, a degree of decomposition in water in a previously wetted condition as measured according to JIS P-4501 of at most 200 seconds, and a wet strength of at least 110 g/25 mm.
Preferably, the water-decomposable fibrous sheet of the invention is a non-woven fabric having been subjected to water-jetting treatment. It is bulky and has a soft feel.
Preferably, the fibrillated rayon constituting the fibrous sheet has a degree of beating of at most 400 cc, and the fibrous sheet is produced in a paper-making process.